Bomechanical overload induces reactive oxygen species that may participate in a broad variety of cellular signaling cascades, and reactive oxygen species may mediate both hypertrophic and apoptotic pathways in cardiac myocytes. Thus, biomechanical regulation of redox balance may play a major role in the pathogenesis of cardiovascular disease. Normally, the cellular cytoplasmic environment is reduced, with many free sulfhydryl groups and relatively rare disulfides; the major ubiquitous reductase responsible for maintaining proteins in the reduced state is thioredoxin. Vitamin D3 Up-regulated Protein 1 (VDUP1) binds to thioredoxin and inhibits thioredoxin activity. Here we present preliminary data on the importance of thioredoxin in cardiomyocyte survival; furthermore, we show that gene transfer of VDUP1 sensitizes cardiac myocytes to oxidative stress-induced apoptosis. We also show that growth factor signaling requires, in some circumstances, elimination of VDUP1 in order to allow thioredoxin activity. Thus, compelling evidence has now emerged that VDUP1, an obscure orphan gene product only a few years ago, is a critical regulator of diverse signaling events due to its direct control of thioredoxin activity. In this proposal, we describe four hypothesis-driven Aims that will define the role of VDUP1 in cardiomyocyte physiology and pathophysiology in vitro and in vivo. We will use a combination of cellular, molecular, genetic engineering, and mouse physiology to address these Aims. Although these experiments are focused on the specific role of VDUP1, they have implications for diverse cellular events beyond cardiovascular disease due to the importance of thioredoxin as both a reactive oxygen species scavenging protein and as a mediator of signal transduction. Aim 1. To explore the mechanisms by which VDUP1 regulates cardiomyocyte viability. Aim 2. To explore the role of VDUP1/ thioredoxin in biomechanically-mediated hypertrophy in cardiac myocytes. Aim 3. To define the role of transient VDUP1 or thioredoxin overexpression using gene transfer in cardiac myocytes under pathophysiologic stress in vivo. Aim 4. To explore whether targeted VDUP1 gene deletion reveals a functional role for VDUP1 in left ventricular hypertrophy and remodeling.